documentclass{article}
usepackage{collcell,xparse,calc}

ExplSyntaxOn
NewDocumentCommand{equalize}{m}
 {
  makebox[widthof{mmmmmm}][s]
   {
    equalize_word:n { #1 }
   }
 }

cs_new_protected:Nn equalize_word:n
 {
  seq_set_split:Nnn l_tmpa_seq {} { #1 }
  seq_use:Nn l_tmpa_seq { hfil }
 }
ExplSyntaxOff

begin{document}

begin{tabular}{|*{3}{>{collectcellequalize}c<{endcollectcell}|}}
hline
abc & defghi & uvwx \
hline
xyz & mmmmmm & vwxyz \
hline
end{tabular}

end{document}